Could you add some unit tests for the new methods added to AuthSourceLdap?  Namely:

AuthSourceLdap#userlist
AuthSourceLdap#includes_cn?
AuthSourceLdap#get_groups (is this actually unused?)
AuthSourceLdap#get_logins
AuthSourceLdap#update_usergroups

I'm happy for you to stub things out, but I think the logic inside them should be exercised.